The present invention relates to structure of an amplifying circuit, and more specifically relates to a particular amplifying circuit of multi-channel input and single-output type.
Conventionally, this type of the amplifying circuit is constructed in view of the multi-channel input and single-output type such that when selecting a particular channel, only the selected channel is biased and the other nonselected channels are not biased.
The drawings are referred to for detailed description of the prior art, in which FIG. 1 shows a schematic structure of the prior art, FIG. 2 shows a circuit of the FIG. 1 prior art and FIG. 3 is a timing chart of switching operation in the FIG. 2 circuit. Referring to FIG. 2, for instance, A channel is initially selected and next, is switched to B channel. In such case, as shown in FIG. 2, a base potential V.sub.A of transistor Tr5 and another base potential V.sub.B of transistor Tr6 are falling and rising, respectively, in illustrated timings. At this time, a collector potential of transistor Tr9 is lowered during a period Td shown in FIG. 3, in which none of the channels is operated, and therefore a collector current of transistor Tr9 is reduced so that a flowing current is reduced through load resistors R.sub.3 and R.sub.4 within an output circuit, thereby causing transient noise.
The above described conventional amplifying circuit of the multi-channel input and single-output type is constructed such that when selecting a particular channel, only the selected channel is biased and the remaining channels are not biased. Therefore, during the switching of channels, one channel which has been selected before the switching is firstly made nonoperative and then another newly selected channel is made operative, thereby causing a time delay in which none of the channels momentarily operates. As a result, a transient noise may be generated at an output terminal to thereby impart bad influence to a circuit subsequently connected to the amplifying circuit.